For all of you who hoped to see the CBT bug vanishing at least with the latest patches and rollups like vSphere 6 update 1a, you have to wait a bit longer.

Once more a critical bug was found in CBT that affects all VM backups, fulls as well as incrementals. The root cause is found (but not published by VMware) and it seems that heavy write I/O on a VM will cause CBT information to be wrong and thus causing incremental backups to contain corrupt data.

Full backups are also affected as CBT is used to find zeroed out space regions to be skipped by the backup solution.

Currently there are only two workarounds: downgrade to vSphere 5.5 (not really an option for all of the users already made the effort to upgrade to version 6) or disable CBT usage inside their backup application. To do this in Veeam foolow this link. This will make backups take longer as all data has to be read from the source disk to compare which blocks have benn changed since the referencing backup but has a lower impact on your environment.A third workaround is mentioned in the KB article from VMware but I don't think that shutting down a VM before taking incremental backups doesn't really make sense.

Probably VMware will release a patch soon but how long is "soon" and how long can you take the risk of having corrupted and therefore unusable backups? So my recommendation is to switch off CBT, make a new full backup of your VMs just to be sure you have a valid base and then wait for another CBT patch from VMware.

VMware has a KB article, to be found here. Subscripe for updates and patches to this bug.